In conventional electrical circuits, electrical relays and switches are employed at points in such circuits where it is desired either to initiate or interrupt (or both) electric current flow through the circuit. In the past, electromagnetic solenoid operated switches and relays have been employed to either close or open the contacts of a power switch or relay in response to a small control signal (low voltage, low current) which initiates either closure or opening of the contact of a larger power rated switch that thereafter controls current flow through the contacts to a circuit being supplied via the switch contacts.
Relays and switches which use piezoelectric drive elements have a number of advantages over their electromagnetic counterparts. For example, a piezoelectric driven relay or switch requires substantially lower current and dissipates very little power during operation to open or close a set of contacts in comparison to an electromagnetic driven device of the same rating. Additionally, piezoelectric driven switching devices have very low mass and therefore require less space and introduce less weight into circuit systems with which they are used. Additionally, piezoeletric driven switching devices possess very short actuation times. Thus, fast acting switching is possible with smaller and lower weight devices that dissipate less power and hence can operate with lower temperature rises if piezoelectric ceramic switching devices are used.
Piezoelectric plate elements may be fabricated from a number of different polycrystalline ceramic materials such as barium titanate, lead zirconate titanate, lead metaniobate and the like which are precast and fired in a desired shape, such as a rectangular-shaped plate. Electrically conducting surfaces in the form of metallized electrodes usually are deposited on the surface of the plates which then are used to apply a polarizing voltage across the piezoceramic plate in order to make them piezoelectric in a chosen polar direction by a prepoling treatment which involves exposing the ceramic plates to a high electric field applied across the metalized electrode while the plates are held at a temperature not far below their Curie point. As a result of this prepolarizing treatment, the plate elongates in the same direction as the applied field. After cooling of the plates and removal of the prepoling field, the dipoles within the ceramic plate which were aligned as a result of the prepoling treatment, cannot easily be returned to their original position and therefore possess what is known as remanant polarization. Thus, the ceramic plates are made permanently piezoelectric whereby the dipoles are permanently enhanced and can convert mechanical energy into electrical energy, and vice versa. The piezoelectric effect is described more fully in a booklet entitled "The Piezoelectric Effect in Ceramic Materials" edited by J. Van Randeraat & R. E. Setterington and published by Philips Golilamperfabriken Eindhoven, The Netherlands, second edition, dated January 1924.
In piezoelectric ceramic materials, the direction of the electrical and mechanical dipole axes depends upon the direction of the original unidirectional prepolarizing high voltage field. During the prepoling process the ceramic plate element experiences a permanent increase in dimension between the poling electrodes and a permanent decrease in dimension parallel to the electrodes. When a DC excitation voltage of the same polarity as the prepoling voltage, but of smaller magnitude, subsequently is applied between the poling electrodes, the element experiences further but temporary expansion in the poling direction and contraction parallel to the electrodes. Conversely, when a DC excitation voltage of opposite polarity is applied to the plate element electrodes, the plate contracts in the poling direction and expands parallel to the electrodes. In either case, the piezoelectric ceramic plate element returns to its original prepolarized dimensions when the later applied excitation voltage is removed from the electrodes.
A number of different piezoelectric ceramic switching devices have been offered for sale in the past having a variety of different configurations. One of the more popular, if not the prevailing structural approach employed in the past, is known as a bimorph bender-type piezoelectric ceramic switch which employs two adjacent piezoelectric plate elements mounted side by side having conductive electrodes, coating their outer surfaces and sharing a common conductive inner surface to form a bimorph bender-tye device. A known commercially available bimorph bender-type piezoelectric ceramic switch is described in an application note copyrighted in 1978 and published by the Piezo Products Division of Gulton Industries, Inc. located in Metuchen, New Jersey and Fullerton, California. If one end of such a piezoelectric ceramic bimorph bender is clamped cantilever fashion, the bender can be made to bend in either direction from its central neutral unenergized condition by application of an energizing potential of either polarity but lower than the prepolarizing potential to one of its conductive outer electrodes. If a suitable value energizing potential of either polarity is applied across only one of the piezoelectric ceramic plate elements of the bender, it enhances dipole alignment of that particular plate element resulting in a shortening and thickening of the plate element. This in turn results in bending of the overall bimorph bender device due to the fact that the two piezoelectric plate elements are physically secured together. By suitable design, the bending action can result in the closing of two switch contacts or other similar effect.
Unfortunately, prior art attempts to provide piezoelectrically driven switch devices have resulted in devices having poor electrical and mechanical performance characteristics. In the case of prior art bimorph bender-type switching devices as described briefly above, they possess severe performance limitations which are founded in the trade-offs between contact force, contact separation, depolarization, retentivity and reliability in service and the uncertainity of contact position due to creep and temperature effects which build up over a period of continued device usage. One such prior art switching device employing a piezoelectric bender-type drive member is described in U.S. Pat. No. 2,166,763 issued July 18, 1939 for a "Piezoelectric Apparatus and Circuits". The piezoelectric bender-type drive member described in U.S. Pat. No. 2,166,763 is comprised by two juxtaposed piezoelectric plate elements having electrodes as described briefly above, and is energized in such a manner that one of the piezoelectric plate elements has the energizing potential applied to it in the same direction as the direction of the prepoling electric field; however, the other piezoelectric plate element has an energizing signal applied thereto of opposite polarity from that of its prepolarizing electric field. As a consequence, the device of U.S. Pat. No. 2,166,763 undergoes long term depolarization of either one or both of the piezoelectric plate elements after a period of usage due to the depolarizing effect of the repeated application of a wrong polarity (out of phase anti-poling direction) energizing signal. The deleterious effect on dipole enhancement of operation in this mode greatly restricts the applied voltage stress and thus the useful work output obtainable with such devices. In addition, the device of this prior art patent possesses a number of other weaknesses sought to be overcome by the present inventon. The same objectionable characteristics are present in a number of different prior art piezoelectric driven bender-type switches and/or relay devices such as the following: U.S. Pat. No. 2,182,340--issued Dec. 5, 1939 for "Signaling System"; U.S. Pat. No. 2,203,332--issued June 4, 1950 for "Piezoelectric Device"; U.S. Pat. No. 2,227,268--issued Dec. 31, 1940 for "Piezoelectric Apparatus"; U.S. Pat. No. 2,365,738--issued Dec. 26, 1944 for "Relay"; U.S. Pat. No. 2,714,642--issued Aug. 2, 1955 for "High Speed Relay of Electromechanical Transducer Material"; U.S. Pat. No. 4,093,883--issued June 6, 1978 for "Piezoelectric Multimorph Switches"; U.S. Pat. No. 4,395,651--issued July 26, 1983 for "Low Energy Relay Using Piezoelectric Bender Elements"; and U.S. Pat. No. 4,403,166--issued Sept. 6, 1983 for "Piezoelectric Relay with Oppositely Bending Bimorphs". In addition to the above prior art patented piezoelectric bender-type switching devices, the textbook "Manual of Electromechanical Devices" by Douglas C. Greenwood published by McGraw-Hill Book Company and copyrighted in 1965 discloses a somewhat similar piezo ceramic switching device on page 64 thereof.
In order to overcome the shortcomings of the known prior art piezoelectric ceramic driven relays and switches such as those listed above, the present invention was devised.